Metal-semiconductor field effect transistors (MESFETs) are much used in modern day electronics, especially high frequency devices employing binary or ternary III-V semiconductors. GaAs, AlGaAs, InGaAs, AlInAs, and other combinations are non-limiting examples of materials commonly used in such devices. Such combination of semiconductor epitaxial layers can create devices with high electron mobility and are often referred to as high electron mobility transistors (HEMT), pseudomorphic high electron mobility transistors (PHEMT) or metamorphic high electron mobility transistors (MHEMT).
FIG. 1 shows a simplified schematic cross-sectional view through generalized MESFET device 20, 20′, 20″ comprising, semiconductor containing substrate 21 extending to interface 19, source-drain regions 23, 24 with their associated contacts 25, 26, and gate 27. Gate 27 commonly comprises Schottky contact forming conductor 271 in contact with portion or region 211 of substrate 21 and overlying conductor 272. Overlying conductor 272 commonly comprises multiple layers of metals provided to serve as a diffusion barrier and to reduce the gate resistance. Portion or region 211 of semiconductor containing substrate 21 is also referred to as Schottky contact forming region 211 and contains or abuts semiconductor channel 22 whose conductivity is modulated by the voltage applied to gate 27, thereby controlling the current flowing between source 23 and drain 24 through channel 22. Substrate 21 may be a monolithic semiconductor or a complex layered structure involving regions formed from different semiconductor materials, as for example but not limited to those mentioned above. Complex substrates 21 are much used for HEMT, PHEMT and MHEMT devices. The construction and operation of such MESFET devices are well known in the art. For example, multilayered HEMT devices are described in commonly held U.S. Pat. No. 6,057,566.
The present disclosure is concerned with the structure and method of formation of gate 27 in relations to underlying Schottky contact forming semiconductor region 211 of substrate 21, and does not depend on the internal details of substrate 21 beneath region 211. Accordingly, substrate 21 may have any combination of internal layers and/or materials desired by the designer, depending upon the particular kind of FET device being fabricated, including but not limited to simple MESFETs, HEMTs, PHEMTs, MHEMTs and other junction field effect devices. Despite their great utility, FET devices of various kinds continue to suffer from a number of problems which limit their operating properties or reliability or both. Accordingly, there is an ongoing need for improved FET structures and manufacturing methods, especially those useful for high frequency applications employing Schottky contacts to III-V containing semiconductors.